1. Field of the Invention
The present invention relates to a two-dimensional shift-register imaging device, and more particularly to an infrared-ray solid state imaging device for converting a two-dimensional image information to a time series electric signal and also to a method for manufacturing the same.
2. Discussion of the Related Art
A conventional infrared-ray solid state imaging device will be first described with reference to FIG. 1, which shows a layout of 2.times.2 imager pixels on a semiconductor substrate. Each pixel has a radiation sensor having a photo-electric conversion layer 1 which accumulates signal charge during an optical integration period. The area for a radiation sensor is called an active area. Each column of the pixels is provided with a vertical CCD (charge coupled device) shift-register 17. The signal charge accumulated by the radiation sensor is transferred to the vertical CCD shift-register 17 through the respective source region 2 of the pixel at the end of the optical integration period.
Each vertical CCD shift-register 17 has a plurality of gate electrodes 25 and 26 provided with, for example, a four phase driving signal by signal lines 25, 26 of polycrystalline silicon layer. Since the photoelectric conversion layers 1 are composed of, for example, a PtSi layer and will be damaged by a high temperature thermal treatment, it is formed after the vertical CCD shift-register 17 and the signal lines 25, 26 are formed and subjected to thermal treatments. Hence, the signal lines 25, 26 are disposed outside the active area.
The signal charges are driven within the vertical CCD shift-registers 17 by the signal lines 25 and 26 in sequence in one direction, for example, downward in the drawing. Then, the signal charges are transferred to a horizontal CCD shift-register (not shown) at the end of each vertical CCD shift-register 17, and then outputted at an end of the horizontal CCD shift-register as a time series signal representing a two dimensional image signal.
A large capacitance is formed between each of the gate electrodes of the vertical CCD shift-registers 17 and the semiconductor substrate, so that each of the signal lines for transmitting the driving signals is required to have a small resistance for enabling a normal transmission of the signal charges through a rapid transmission and avoidance of a wave distortion of the driving signals. To reduce the resistance of the signal lines usually requires a large area for the signal lines.
It is especially required for an infrared-ray solid state imaging device to have a large ratio of the active area to the pixel area including the active area and the inactive area which includes the area for a respective part of the vertical CCD shift-registers 17 and for the signal lines 25, 26. With the conventional infrared-ray solid state imaging device, however, it is difficult to employ a large ratio of the active area to the pixel area due to the large area for the signal lines. It is noted that the above-mentioned conventional structure is applicable to sense a visible image and convert it into electrical information.